Traditional underwater acoustic sensors are pressure sensors (e.g., hydrophones) responsive to oscillating pressure in the field of the acoustic wave. Pressure is a scalar quantity, so a single hydrophone (smaller than the acoustic wavelength) has no directionality. Unlike hydrophones, vector sensors are responsive to water oscillatory velocity (or pressure gradients) associated with the same acoustic wave. Because velocity is a vector, the vector sensor has directionality even if its size is much smaller compared to the acoustic wavelength. This may not be a big advantage at relatively high acoustic frequencies (tens of kHz), but it has an unsurpassed advantage at low frequencies (tens-thousands of Hz) and, especially, at ultra low frequencies in the range of a fraction of a Hz to tens of Hz.
To measure particle velocity in the water, conventional vector sensors (or particle velocity sensors) are designed to be neutrally buoyant in the water column. Developing a highly sensitive, low-noise vector sensor in a small, neutrally buoyant package is extremely challenging. It becomes even more challenging for the ultra low frequency range. One reason accelerometers used in conventional sensors are not suitable for sensitive measurements at ultra low frequencies is that acceleration is proportional to f*v, where v is the particle velocity and f is the frequency. As the frequency decreases, the accelerometer's sensitivity decreases accordingly. It will be more advantageous to measure displacement, x, rather than acceleration, as the displacement is proportional to v/f. That is, for the given particle velocity, the displacement is increased as the frequency goes down.
Another challenging problem is the suspension of the sensor in a water column. The conventional sensor is configured as a neutrally buoyant body containing an inertial measuring element such as an accelerometer. Yet the sensor should be fixed at a particular location in the water column using some sort of suspension element. However, the suspension element restricts the free motion of the neutrally buoyant body which interferes with measurements. The neutrally buoyant body is also directly exposed to water currents which force the body to float away from its designated location, while the suspension element restricts this forced motion. All of these factors causes additional noise and interference with the sensor operation, especially at low frequencies.